This invention relates to circuit board techniques, and more particularly to a circuit board including through-holes formed by punching using a punching pin and a through-hole conductive section formed by filling each of the through-holes with a conductive paste and a method for manufacturing the same.
An increase in current flowing through a circuit on a circuit board requires an increase in width of a circuit pattern, as well as an increase in current capacity of each of through-hole conductive sections or a reduction in electrical resistance of each through-hole conductive section. In a conventional circuit board, the through-hole conductive section increased in current capacity is formed by plating. For example, Japanese Patent Publication No. 24109/1967, Japanese Patent Application Laid-Open Publication No. 88079/1974, Japanese Patent Application Laid-Open Publication No. 9755/1975, Japanese Patent Application Laid-Open Publication No. 40560/1976, Japanese Utility Model Application Laid-Open Publication No. 52857/1976, Japanese Patent Application Laid-Open Publication No. 50560/1977, Japanese Patent Application Laid-Open Publication No. 59860/1977 and Japanese Patent Application Laid-Open Publication No. 97895/1983 each disclose techniques of forming through-hole conductive sections increased in current capacity by plating. Such formation of the through-hole conductive section by plating causes plating liquid to be impregnated in an insulating substrate of a circuit board, leading to a deterioration in strength of the insulating substrate. In order to avoid the problem, the insulating substrate is conventionally made of a glass-epoxy material, a composite material consisting of glass and resin fiber, or the like. Unfortunately, such a material for the insulating substrate is expensive, resulting in the conventional circuit board provided with the through-hole conductive sections increased in current capacity being significantly increased in cost.
Conventionally, an insulating substrate reduced in cost may be provided in the form of a paper base insulating substrate made by impregnating a paper base material with a thermosetting insulating resin material such as phenol resin or the like. Unfortunately, such a paper base insulating substrate causes plating liquid to be impregnated in the paper base material, leading to a failure in formation of the through-hole conductive sections by plating. In view of the fact, in a circuit board formed of such a paper base insulating substrate and provided with the through-hole conductive sections, the through-hole conductive sections each are conventionally formed of a conductive paste. More particularly, first the paper base material is impregnated with thermosetting insulating resin to provide the insulating substrate, which is formed on both surfaces thereof with a copper foil by cladding to prepare a copper-clad insulating substrate. Then, the insulating substrate is thrust from a side of a front surface thereof by a punching pin, to thereby be formed with through-holes. Subsequently, a conductive paste prepared by adding a conductive powder material to thermosetting insulating resin is filled in the through-holes by printing and then cured, so that the through-hole conductive sections may be provided.
However, as noted in the art, the through-hole conductive sections thus formed by filling the through-holes with the conductive paste each are reduced in current capacity as compared with the through-hole conductive section formed by plating. More particularly, the former through-hole conductive section is increased in resistance as compared with the latter one. Thus, in the circuit board increased in current capacity requires, it is required to form each of the through-hole conductive sections using an expensive insulating substrate such as a glass-epoxy substrate and by plating.
A reduction in cost of the circuit board has been increasingly demanded. Unfortunately, the circuit board formed with the through-hole conductive section by plating fails to satisfy the demand.